Application of laser technology to such diverse fields as geophysical measurement (e.g., land surveying, range finding) and long distance communication (e.g., electrooptic communication networks) has created a need to improve the performance of laser transmitters to equal the demands in those arts for precision. Precision is a function of the accuracy and resolution provided by an instrument used to make a measurement. In an optically pumped, continuous-wave pulse laser transmitter, precision requires generation of very short laser emission pulses readily susceptible to accurate resolution by a receiver. Accurate resolution is enhanced by a laser transmitter providing light pulses with narrow pulse width and stable temporal waveforms.
Basically, the laser medium of a laser transmitter is an active device which exhibits a gain phenomenon. The medium serves as the active component of an oscillator called a resonator. In an optically pumped laser, a flash lamp, electrically driven into ionization, transfers energy in the form of intense bursts of light to the laser medium. The quantity of energy transferred must exceed the base threshold of the medium to excite the medium to emission. The waveform of the laser emission pulse closely resembles the waveform of the burst of light from the flashlamp. A medium pumped by a single spike of light energy, for example, will emit a pulse exhibiting a gaussian waveform.
Any small noise source, whether internal or external to the laser medium, manifests itself as instability in the laser resonator, and has the ability to upset the steady state dynamic condition of the laser transmitter. Instability in the resonator causes emission of multiple, non-uniform laser pulses. One consequence of this is that when repetitively pumped by a flash lamp driven by a pulse forming network, particularly a multi-mesh network, successive pulses emitted by the lamp, and thus the laser resonator, tend to unpredictably differ in such waveform characteristics as amplitude and pulse width. There are two causes for this. First, the energy emitted by a flashlamp is very sensitive to changes of impedance. The lamp impedance changes drastically (by several orders of magnitude) with variations in the lamp current. Second, pulse forming networks, primarily one or more parallel stages each with an energy storage capacitor and an inductor coupled across the electrodes of a lamp, inherently exhibit a ripple in the amplitude of current provided to the lamp. The inherent ripple is compounded in multi-mesh type pulse forming networks. To avoid instability in the resonator, the amount of ripple in the amplitude of the discharge current pulse driving the flashlamp would be limited to less than one-half of one percent. Generally, multi-mesh pulse forming networks exhibit between two and five percent ripple in the discharge current pulse.
Pulse forming networks previously used to address the need for providing temporally uniform pulses to the laser medium and, therefore, the flashlamp pumping the medium, have sought to provide rectangular discharge pulses to the flashlamp by modifying the exponential decay of the discharge pulse. These networks include a silicon controlled rectifier which shorts the flashlamp when fired by a time delay stage set by the same initializing pulse that fires the lamp. Another prior art network relies upon a mismatch of impedance between the network and the lamp to cause a reversal of polarity shortly after the lamp is fired, thereby assuring a quick turnoff of a switching device located between the network and the lamp. Neither exemplar addresses the problem of maintaining the impedance of the flashlamp and thus, the amplitude of the power transferred to the laser resonator, constant during the discharge pulse.